Recent comments in /f/askscience

SW_Zwom t1_j2sdh2u wrote

Reply to comment by Weed_O_Whirler in If an object is placed at the end of of a VERY long bar that is connected to an axel, Could rotating the bar close to the axel make the object at the other move extremely fast (like light speed)? by Swade060504

I was actually asking myself this: If the rod did not bend at all - could we even move it? I mean without any pressure wave travelling through the material it could not accelerate. So wouldn't perfect rigidity require an infinite speed of sound inside the material? That would make it impossible, even in a "ideal" calculation...

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Weed_O_Whirler t1_j2sb2h7 wrote

Reply to If an object is placed at the end of of a VERY long bar that is connected to an axel, Could rotating the bar close to the axel make the object at the other move extremely fast (like light speed)? by Swade060504

Everything you're saying holds up, except for this one thing:

> a beam that cannot flex and is incredibly long

In physics, we often times talk about "rigid bodies" and make the assumptions that they are infinitely stiff, but that's just a "small thing, moving slow" assumption- where they appear to be completely rigid. In real life, materials are not completely rigid.

Now, you might think "that's just an engineering problem, we can just design something to be really stiff. But you can't. Information- including compressions and rotations, travel through a material at that material's speed of sound. Which makes sense if you think about it, sound waves are compressions passing through a material.

So, if you had a really long rod, and you started rotating it faster and faster, the rod would start to bend and then shear. And that's not an engineering problem- it's a real physics one.

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Pink_Axolotl151 t1_j2s7n6z wrote

Reply to How are birth control failure rates even calculated? by [deleted]

Also, keep in mind that the publicized control failure rates represent failure over a 1-year period, but the longer you go out, the higher the failure rate. For example, for male condoms, the failure rate of perfect use is 2% in 1 year, but 18% over a 10-year period. For typical use, it’s 18% after 1 year and a whopping 86% after 10 years. This article has interactive charts that show how the failure rates with perfect and typical use are compounded over time for several different types of birth control:

https://www.nytimes.com/interactive/2014/09/14/sunday-review/unplanned-pregnancies.html

I know that’s not exactly what you asked but I like to throw out this information because I think it’s important.

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Alittlebitmorbid t1_j2s24qf wrote

Reply to comment by Snizl in Are there any examples of natural hybridization, or any possibilities of this occurring in nature? by Owlsthirdeye

Both differ genetically enough from the domesticated variants and took different paths thousands of years ago. In fact it is not sure the Przewalskis were from a domesticated group. They also show a huge lot of characteristics common in wild horses and may be a mix of the last remainders of wild living horses in Europe and domesticated ones. But there are enough other examples. In birds there are about 4000 proven examples of hybridization, half of it due to captivity, the other half occuring naturally, but the numbers are estimated to be higher as it is not always possible to identify wild living hybrids. Also we humans are hybrids, there's still Neanderthal DNA found in us.

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